Gas chromatographic &#34;in-column&#34; spectroscopic analysis

ABSTRACT

A chemical detector for rapid, simultaneous detection of multiple chemicals including chemical warfare agents, toxic industrial chemicals, and explosives having one or more gas chromatography columns each with a chemosorbent or a chemo-reactive stationary phase and an infrared-transparent base, a bright infrared light source, a mechanism to direct the light source to any point along any of the columns, and an infrared sensor. Another disclosed detector has one or more gas chromatography columns each on the surface of a substrate having at least one infrared-transparent waveguide pattern, a bright infrared light source, and at least one ring resonator for each column, where each ring resonator is coated with a chemosorbent or a chemo-reactive stationary phase, and where each ring resonator spectroscopically probes the stationary phase. Also disclosed are the related methods for chemical detection.

PRIORITY CLAIM

The present application is a continuation application claiming thebenefit of U.S. application Ser. No. 14/208,088, filed on Mar. 13, 2014by R. Andrew McGill et al., entitled “Gas Chromatographic ‘In Column’Spectroscopic Analysis,” which was a non-provisional applicationclaiming the benefit of U.S. Provisional Application No. 61/788,723,filed on Mar. 15, 2013 by R. Andrew McGill et al., entitled “GasChromatographic ‘In Column’ Spectroscopic Analysis,” the entire contentsof each are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to IR detectors and, morespecifically, to IR detectors using a gas chromatographic in-columnspectroscopy.

Description of the Prior Art

A significant remaining challenge today in the chem-bio defensecommunity is the simultaneous and reliable detection of a large numberof chemicals and types covering all the chemical warfare agents (CWAs)and toxic industrial chemicals (TICs) in a “point” detectionapplication. The development of a detector with suitable analyticalfidelity can be reduced to the development of strategies or techniquesto best exploit differences in analyte physicochemical properties.

IR spectrophotometry is an established lab fixture providing richmolecular information content, however performance is degraded forcomplex mixtures and traditional hardware suffers from relatively lowsensitivity.

Fan et al. have reported in-column sensing. U.S. Patent ApplicationPublication 2013/0169970 by Fan et al. (Jul. 4, 2013) and Reddy et al.,“Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gaschromatography,” Lab Chip, 12, 901-05 (2012). However, Fan et al.perform non-spectroscopic sensing and focus on measuring refractiveindex changes in a sorbent coated sensor embedded in the gaschromatography (GC) column. This approach lacks the selectivity providedby an IR absorption spectroscopic sensing approach. Moreover, thisapproach uses discrete sensors that do not sense at all points along thecolumn.

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems are overcome in the present invention whichprovides a chemical detector for rapid, simultaneous detection ofmultiple chemicals including chemical warfare agents, toxic industrialchemicals, and explosives having one or more GC columns each with achemosorbent or a chemo-reactive stationary phase and aninfrared-transparent base or column, a bright infrared light source, amechanism to direct the light source to any point along any of thecolumns, and an infrared sensor. When multiple columns are used, thecolumns may be operated in parallel. In some configurations a singlecolumn with different regions employing different sorbent stationaryphases is advantageous for selectivity and sensitivity purposes. In apreferred embodiment, at least one stationary phase comprises acarbosilane polymer with hydrogen bond (HB) acidic functionalization.Other examples include HB basic polymers, polar, polarizable andnon-polymer polymers and organometallic materials offering transitionmetal coordination site chemistries. The infrared-transparent base canbe an attenuated total reflection (ATR) crystal. The bright infraredlight source can be a tunable infrared quantum cascade laser (QCL). Theinfrared sensor can be preceded by an integrating sphere to remove thechanges in the infrared signal due to slight direction changes of thelaser beam when examining different points along the column.Alternatively, an IR focal plane array (FPA) can be used to image allthe channels simultaneously, as the laser wavelengths are tuned. In thisconfiguration, the laser light would illuminate all of the columns, asopposed to being focused as is the case when using a single elementdetector. The detector system can also include a preconcentrator devicebefore or at the head of the GC columns. Another disclosed detectorsystem includes one or more GC columns each on the surface of asubstrate having at least one infrared-transparent waveguide pattern, abright infrared light source, and one or more ring resonators for eachcolumn, where each ring resonator is coated with a chemosorbent or achemo-reactive stationary phase, and where each ring resonatorspectroscopically probes the GC stationary phase. Also disclosed are therelated methods for chemical detection.

The present invention allows the detection of a wide range of chemicalswith a wide range of vapor pressures present in the air as complexmixtures. This has only been previously possible with relatively largeinstruments, whereas the present invention allows small hand-heldconfigurations to be realized. The present invention allows analyses tobe made at the head of the GC column in a preconcentrator or collectiondevice and, while analytes progress along a GC column, by probing an IRtransparent side of the column, rather than having to wait for end ofcolumn elution. This reduces unnecessary wait time and increasessensitivity by avoiding unnecessary peak spreading. This allowsdetections to be monitored during column separations and an alarm to beachieved before end of column elution. The optical interrogation allowsrapid analysis at any point along different columns by directing thelight appropriately. By combining the elements of column separation andIR spectroscopy analysis (with a tunable bright IR source) and arrays ofstationary GC phases with IR sensors, the present invention offers thepotential for high analyte discrimination with short analysis times. Thepresent invention incorporates an ultra-bright monochromatic tunable IRsource (e.g. QCL) which allows selective probing of analytes. Incontrast, conventional IR spectroscopy uses a weak IR source generatingIR light over many wavelengths which provides low sensitivity and poorselectivity derived from the incident IR light. Analysis occurs asanalytes propagate along a column, but conventional gas chromatographydetectors may also be used at the end of the column. So detectionalgorithms will be responsible for analyzing data during analytepropagation through the column and possibly for data collected from asensor at the end of the column. Algorithms include, but are not limitedto: least squares regression techniques (including non-negative LS),SVM, matched and adaptive sub-space detectors, spectral match filtering,adaptive boosting, neural networks, PCA, ICA, and greedy pursuitalgorithms. The data may take the form of hyperspectral image cubes, inwhich case a method specific to that form (e.g., the sub-space detector)should be used.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an array of GC columns deposited on a prism used in an ATRconfiguration and probed by a tunable ultra-bright IR QCL light sourceto measure “in-column” IR absorption spectra as a function of incidentwavelength and location along the GC column.

FIG. 2 shows an array of serpentine GC columns deposited on an ATRcrystal. Analyte carried by carrier gas flow is trapped at the head ofeach column in a preconcentrator section. This may be a relatively thicksorbent coated area and may be separately temperature controlled fromthe rest of the GC column with a built-in micro heater and heated torelease analyte for GC injection. The stationary phase trace may becapped with a variety of passivated structures to seal with the ATRcrystal surface and allow carrier gas flow along the stationary phaselength.

FIG. 3A shows a differential FTIR spectra for SiFA4H-Nitrobenzene HBadduct. FIG. 3B shows a differential FTIR spectra for SiFA4H-DMMP HBadduct.

FIG. 4 shows a SiFA4H Solutochromic hydroxyl shift (MOH) correlated withvapor adduct HB basicity (B) and the structure of SiFA4H.

FIG. 5 shows an array of IR ring resonators positioned along a GC columnor GC columns and coated with different stationary sorbent phases.Evanescent field absorption spectroscopy is used to probe each sorbentcoated micro-ring resonator over a range of incident wavelengths.Preferred interrogation wavelength range is 1-10 microns. Ringresonators are operated to allow in-column monitoring of solute oranalyte as it progresses along the column(s).

FIG. 6 shows an ATR-based sensor using diffraction grating.

FIG. 7 shows an ATR-based sensor using a MEMS light source and abandpass filter.

FIG. 8 shows a Raman-based sensor.

FIG. 9 shows sorbent polymer separation spectroscopy using an array ofpolymer films on an ATR prism.

FIG. 10 shows a multi GC-IR-ATR-Spectroscopy concept of pressing andattaching a pre-coated GC cartridge onto the prism.

DETAILED DESCRIPTION OF THE INVENTION

The purpose of the present invention is to provide a general frameworkto optimize molecular affinity and analyte discrimination for a largegroup of chemicals by employing a set of sorbent chemistries,chromatographic separations and “in-column” detections using infraredlight to probe and access molecular information.

The analytical separation power of multimodal gas chromatographic (GC)columns, operated in a parallel format, is combined with IR absorptionspectroscopy used to monitor analytes during their chromatographicprogression “in-column”. Sensitivity and selectivity are augmented byincorporating selective sorbents as GC stationary phases to target CWAsand TICs through reversible binding analyte or chemical reactionsbetween an analyte and a stationary phase. An ultra-bright tunable IRlaser, such as a quantum cascade laser (QCL) or other bright IR lightsource provides a dramatic increase in photon density to probe thechemistries involved. By design, IR interrogation and analyte detectionis enabled at all points along a column during the GC analysis. As shownin FIG. 1, a moveable mirror can direct the infrared light source. Also,a focusing lens and mirror can direct the light source. QCL spot sizemay be localized to the stationary phase or column width and monitoredwith a single element IR sensor as a function of column scan, or alarger QCL spot size can bathe the entire GC array and an arrayed IRsensor can simultaneously monitor all column points. To achieve this, anIR-transparent column base (e.g. polished ATR crystal) is incorporatedin the column design and bonded to a bridging structure with a gas tightflow manifold (FIGS. 1 and 2). For less complicated analyte samples,suitable high detection fidelity may be achieved at or near a GC columninlet whereas for more complex samples this may occur further along acolumn with additional separation. In-column analyses allow optimumdetection to be achieved or a preliminary rapid detection call to bemade without waiting for complete column elution with subsequentdetections along a column providing increasing detection confidence iffurther separation of analytes is achieved.

IR Spectroscopic Properties

The characteristic IR absorption bands of the analytes and sorbents asseparate or bonded materials are the features used to help identifyindividual molecules or intermolecular analyte-sorbent bonds. FIGS. 3Aand 3B illustrate the effect of the vapor-polymer HB for the —OH stretchin SiFA4H, a HB acidic carbosilane sorbent. (Among the differentialsignal components could be the heat of sorption, which would increasethe FTIR signal. While heats of sorption are non-selective, thephotothermal signatures emitted provide spectral information.) The —OHIR stretching band shifts by hundreds of wavenumbers depending on thevapor-sorbent HB strength. FIG. 4 shows that the degree of shift is wellcorrelated with the HB basicity of the vapor or gas.

One important class of TICs, the di-halogens (e.g. chlorine), are not IRactive as separate species but are readily reacted to specific polymerfunctionalities that provide suitable IR cross sections.

IR Spectroscopy measures the vibration modes of molecules in the IR(2-16 μm) region of the spectrum. A portion of this spectrum is theso-called fingerprint region (6-14 μm) where complex signatures ofmolecules are present that make this region particularly suitable forsensor applications with high selectivity. Unfortunately, this is also aspectrally busy region with overlapping peaks and the performance offitting algorithms is diminished for mixtures. To overcome this, insteadof a single measurement, IR spectra are measured in multiple sorbentpolymers. Due to the variations of analyte affinity towards differentsorbent layers, the present invention provides several independentmeasurements with varying mixture concentrations. When analyzed inconjunction with spectra from along GC columns, detection algorithmperformance can be greatly improved. The optical absorbance for a givensorbent polymer p at the beginning of the column (assuming N_(c) analytecomponents in the mixture) is given by: A_(p)(λ)=α_(p)(λ)+Σ_(i=1)^(N=)c_(t) ⁰K_(i,p)(α₁(λ+Δα_(pi)(λ)) where α_(i)(λ) and α_(p)(λ) are theIR absorbance of the analyte and the polymer before sampling, K_(i,p) isthe partition coefficient for analyte i in polymer p and Δα_(pi)(λ) isthe change in polymer optical absorbance due to chemical reactions orsolutochromic shifts. c_(i) ⁰ are the concentrations of components inair and are the only fitting parameters. For points along the column,standard GC equations apply.

ATR IR Spectroscopy

The utility of IR spectroscopy is harnessed in an attenuated totalreflection (ATR) configuration, which takes advantage of the evanescentfield of the light that extends ˜1 μm from a reflected prism surface(FIG. 1). This small interaction depth is an ideal fit for a thinsorbent layer required for GC analysis. Other sorbent coated opticalconfigurations for this invention are possible and are described below.

Ideally the spot size of the incident IR light would match the width ofthe stationary phase in the GC column and form a thin rectangular shapewith the thin side of the rectangle directed along the GC column length.

Instead of employing an ATR crystal approach, the more straightforwardtransmission of IR light through a sorbent stationary phase can beutilized. Moreover, it is possible to include other monitoringapproaches in addition to IR probe sensing (e.g. refractive indexmonitoring).

Multidimensional Gas Chromatography

To consider the wide range of CWA and TIC analytes of interest, multiplecolumns operated in parallel are included. Each column is coated with adifferent sorbent stationary phase. Chemosorbent stationary phases areselected to target nerve, blister, blood and other CWAs and selectedTICs. Chemoreactive stationary phases are selected for those TICs notsuitable for effective chromatography. In the latter case, the GC columnstill serves the purpose of separating chemicals not of interest awayfrom the analyte of interest. It also functions effectively as adosimeter record for the TIC

The GC columns may be operated in parallel or sequentially.Additionally, a single column may include more than one stationary phasecoating either coated in series fashion or on different interiorsurfaces of the column structure to form analyte competitive surfaceswhich can improve the selectivity of the chromatography and detectionprocess.

The stationary phase may be deposited as a single “strip” to allow a gapbetween the side wall of the column. This prevents any sorbentstationary phase pooling in any column crevice or angular structures.Sorbent pooling leads to undesirable effects on GC separationperformance with significant peak tailing.

The GC column can be fronted by a sorbent coated preconcentrator orsorbent coated “focusing” device which collects analyte before or at theinlet end of the GC column and is actuated by heating to release anysorbed vapors and gases into the GC column. This may simply comprise athicker coated area at the head of the GC column which is separatelyheated to allow rapid thermal ramping to desired injection temperatures.Alternatively, a cryogenic trap may be positioned before or at the inletend of the GC column.

Additionally, there can be a “smart” control mechanism upstream of theGC. The use of infrared absorption spectroscopy (IRAS) or Ramanspectroscopy can be used to probe a preconcentrator or injector zone sothat the GC doesn't probe or perhaps even operate until there issomething of interest collected. If microfabricated, the GC could bequickly brought up to operational temperature in seconds only whenneeded to save on power.

After injection into a column, the column temperature may be controlledto allow elution of some analytes but essentially no column travel forother analytes. IRAS can be used to probe near the entry of the columnto monitor analytes that don't move down the column and actuate heatingif necessary to move analytes that are stationary at the head of thecolumn. A “smart” GC temperature ramping system control could be usedwithout the need for predefined isothermal column conditions,temperature ramp rates, and target temperatures. The controlled heatingelements may comprise resistive cartridges, meander heater traces, andthermoelectric devices. These heating elements may be near and along thecolumns for changing the column temperature during analysis according toa prescribed schedule or as part of an active feedback loop based on thedetector response during measurement.

The column may also have a carrier gas flow rate control mechanism. Ifmore time is desired to examine one or more bands of chemicalprogressing along a column, a command can be sent to reduce the carriergas flow rate or turn it off to halt any further progression until thecarrier flow is started again.

Chemosorbent Materials

A range of commercial and custom sorbents can be used as stationaryphases are including a set of hypersorbent HB acidic materialspreviously developed at NRL for nerve and blister CWAs. Several of theseHB acidic carbosilane polymers have a demonstrated pedigree withmillions of thermally cycled applications in air, demonstrating nomeasurable change in sorption properties. This is an importantperformance metric for a polar GC stationary phase operating with air asa carrier gas.

TIC Chemo-Reactive Materials

A significant number of the TICs are permanent gases under ambientconditions and because of their high vapor pressure, partitioning intopolymer phases is relatively low. Other stationary phase candidates toconsider include those which emphasize reactivity as either oxidizing orreducing agents and their Lewis acid or base properties. A number of thehigh threat TICs (HCN, HF, HCl, HBr, H₂S, HNO₃) exhibit significantLewis acidity as gas phase species and bind well to surfaces withcomplimentary Lewis base properties such as alumina (Al₂O₃). Theseadducts provide IR signatures for identification. HNO₃ has also beenshown to form nitrate salts when exposed to zinc chloride; however, thispresents a more difficult path for regeneration for multiple use.Another TIC, BF₃, reacts in air to form HF so it may also be detected bythe HF adduct to a Lewis base. The chlorine, bromine and fluorinedihalogen TICs are reactive to a number of chemistries under ambientconditions including the alkene double bonds forming dihalidestructures. Polybutadiene is a suitable polymer for this purpose. Thehalogenated products provide suitable IR signatures for detection. OtherTICs such as ammonia are naturally present at low concentrations in theenvironment and therefore may not be useful candidates for such reactionschemes; however, transition metal coordination chemistries for NH₃ andAsH₃ are paths to reversible IR signatures.

When using reactive sorbent chemistries that chemically bond to TICs orother hazardous chemical, changes in chemical bonding in the sorbent canbe monitored. Once reacted, that portion of the GC column is not capableof reacting with the analyte of interest unless a regeneration protocolis available. The GC-IRAS system could be instructed to ignore thereacted zone and monitor further along the column (this happensinherently in a “differential” spectroscopic approach by renormalizingthe start point). Chromatography is effected in the column, and theamount of reacted sites quantifies the analyte in a dosimeter fashion.

Waveguide Evanescent Field Spectroscopy

Instead of placing the GC column(s) on an ATR crystal, the columns canbe located on a substrate that has had waveguides patterned along itssurface (FIG. 5). These waveguides can be made of material that istransparent to infrared radiation, and can be patterned to bring theradiation to specific locations at the GC columns for high spatialresolution spectroscopic probing. Either tunable infrared lasers withsingle element detectors or broadband infrared sources with spectrallyresolved detectors can be used with these waveguides. As long as thewaveguides are thinner than the wavelength of the infrared radiation,strong evanescent fields will exist within the sorbent material.Microcavity waveguide resonators, such as Fabry-Perot cavities ormicrorings can be used to increase the effective optical interactionlength without increasing the footprint size of the infrared probe. Theevanescent field above the waveguide will spectroscopically probe thesorbent material in much the same way as ATR-FTIR spectroscopy.Embedding mid-IR transparent microrings under sorbent coatings in microGC columns allows for in-column spectroscopic analysis.

Alternatives

In addition to coating the GC sorbent stationary phase on the ATRcrystal or optical waveguide structures, a second stationary phase canbe coated on another interior face or opposite side of the columnstructure. This then provides an analyte competitive sorbent phase tothe optically probed sorbent phase. Analyte in the gas phase is thendistributed between the 2 different sorbents. By appropriate selectionof sorbents the chemical selectivity of the sorbent coated on the ATRcrystal can be substantially improved.

The chemical detector can also comprise an ultraviolet (UV) or visiblelight source and detector directed at the stationary phase through atransparent column for examining the reflectance spectra andfluorescence spectra of the analyte bound to a chemosorbent orchemo-reactive stationary phase along the column. There are some dyesthat change color in the visible light range when chemicals bind tothem. These can be dissolved in a stationary phase and then when achemical is sorbed to the polymer it binds with the dye and changescolor in the visible or UV light range.

In addition to gas chromatography, a liquid chromatography configurationcould also be used.

FIGS. 6-10 show alternative configurations. These may be implementedwith or without the in-column GC configuration. FIG. 6 shows anATR-based sensor using diffraction grating. FIG. 7 shows an ATR-basedsensor using a MEMS light source and a bandpass filter.

FIG. 8 shows a Raman-based sensor. In addition to probingsorbent-analyte interactions with light, a SERS configuration may beused where nano metal particles are embedded in the sorbent stationaryphase and probed with a RAMAN laser and a suitable light detector.

FIG. 9 shows sorbent polymer separation spectroscopy using an array ofsorbent polymer films on an ATR prism. Although not shown, platinummeander traces for joule heating (desorption) can be deposited on thesurface of the prism prior to sorbent coating. These traces can be usedfor closed-loop temperature control.

FIG. 10 shows a multi GC-IR-ATR spectroscopy concept using analternative sorbent polymer deposition. A single GC column has a seriesof channels, which are completely coated. This GC column becomes adisposable part. To replace it, one would just wipe off any old polymerresidue off of the prism and press onto the prism a new GC “cartridge”already coated with stationary phase. The grooves in the channel allowfor air to flow, and the coated ridges make intimate contact with theprism and can be interrogated using ATR spectroscopy or bridgeresonators.

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” is not to beconstrued as limiting the element to the singular.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A chemical detector for simultaneous detectionof multiple chemicals including chemical warfare agents, toxicindustrial chemicals, and explosives, comprising: one or more gaschromatography columns, wherein each column comprises one or morechemosorbent or chemo-reactive stationary phases, and wherein eachcolumn has an infrared-transparent base; an infrared light source; amoveable mirror to direct the light source to the more than one gaschromatography columns, wherein the light source can be directed to anypoint along any of the columns or to a preconcentrator or collectordevice, wherein in-column chemical detection can occur at all pointsalong every column; and an infrared sensor.
 2. The chemical detector ofclaim 1, wherein when there is more than one column, each column isoperated in parallel.
 3. The chemical detector of claim 1, wherein atleast one stationary phase comprises a carbosilane material withhydrogen bond acidic functionalization.
 4. The chemical detector ofclaim 1, wherein the infrared-transparent base comprises an attenuatedtotal reflection (ATR) crystal.
 5. The chemical detector of claim 1,wherein the infrared light source comprises one or more optionallytunable infrared lasers, and more preferably the bright infrared lightsource comprises one or more quantum cascade lasers (QCLs).
 6. Thechemical detector of claim 1, additionally comprising a focusing lensused with the moveable mirror to direct the light source.
 7. Thechemical detector of claim 1, additionally comprising an analytepreconcentrator comprising sorbent coated structures positioned beforeor at the inlet end of at least one of the columns.
 8. The chemicaldetector of claim 1, additionally comprising one or more independentlycontrolled heating elements along one or more columns.
 9. A method forsimultaneous detection of multiple chemicals including chemical warfareagents, toxic industrial chemicals, and explosives, comprising:injecting a vapor-phase analyte sample into one or more gaschromatography columns, wherein each column comprises one or morechemosorbent or chemo-reactive stationary phases, and wherein eachcolumn has an infrared-transparent base; directing an infrared lightsource to a point at one of the columns, wherein the light source can bedirected to any point along any of the columns or to a preconcentratoror collector device, wherein in-column chemical detection can occur atall points along every column; and using an infrared sensor to detectfor the presence of chemicals.
 10. The method of claim 9, wherein whenthere is more than one column, each column is operated in parallel. 11.The method of claim 9, wherein at least one stationary phase comprises acarbosilane material with hydrogen bond acidic functionalization. 12.The method of claim 9, wherein the infrared-transparent base comprisesan attenuated total reflection (ATR) crystal.
 13. The method of claim 9,wherein the infrared light source comprises one or more optionallytunable infrared lasers, and more preferably the bright infrared lightsource comprises one or more quantum cascade lasers (QCL).